1. Field of the Invention
The present invention relates to a method of automatically feeding a wire to an electric discharge machine which effects the discharge machining to a work by giving an electric discharge energy between the work and a wire electrode.
2. Description of the Prior Art
In a conventional electric discharge machine, for example, the work such as a super hard alloy and a hardened steel are machined using a portion where the wire electrode runs, and the wire is usually fed by an automatic wire feeding device. Conventional automatic wire feeding devices can be divided into those of a pipe guide system, those of a water stream guide system, and those of a start hole guide system.
FIG. 9 illustrates an automatic wire feeder based on the start hole guide system. The automatic wire feeder according to the start hole guide system is achieved by positioning the wire feeder at a start hole 26 formed in a work 8, penetrating a wire electrode through the start hole 26 via a wire feeder device 28, a feeder pipe and an upper guide 32, and pulling out the wire electrode 1 by the lead rollers located on the downstream side of a lower guide 29. The start hole system has a merit in that the wire can be penetrated even when the start hole 26 has a small diameter as well as a demerit in that the wire electrode itself must be composed of a hard wire.
FIG. 10 illustrates an automatic wire feeder based on the pipe guide system. The automatic wire feeder according to the pipe guide system is achieved by positioning the wire feeder at a start hole 26 formed in the work 8, passing a pipe 27 that works as a guide through the start hole 26, actuating the wire feeder device 28 to pass the wire electrode 1 through the pipe 27, and pulling out the wire electrode 1 by the lead rollers positioned on the downstream side of the lower guide 29. The pipe guide system has a merit in that the feeding of wire electrode can be achieved maintaining a very high success rate as well as a demerit that the start hole 26 must have a large diameter since the pipe 27 that serves as a guide must be passed through the start hole 26.
FIG. 11 illustrates an automatic wire feeder based on the water stream guide system. The automatic wire feeder based on the water stream guide system is achieved by positioning the wire feeder at a start hole 26 formed in the work 8, injecting a working fluid 31 into the start hole 26 through a working fluid guide 30, passing the wire electrode 1 through the start hole 26 that is fed by the wire feeder device 28 accompanying the force of injection of the working fluid 31, and pulling out the wire electrode 1 by the lead rollers positioned at the downstream side of the lower guide 29. The water stream guide system has a problem in that the wire feeding is achieved at a very poor success rate.
According to the above-mentioned systems, however, in case the wire electrode 1 is broken during the discharge machining, the wire electrode 1 is coupled together, i.e., the wire is automatically fed again by necessarily returning the procedure to the start hole 26 which is the start point of machining. That is, feeding of the wire electrode 1 is achieved by moving the work table relative to the wire head to return the wire feeding portion of the wire head to the start hole 26 formed in the work, passing the wire electrode 1 through the start hole 26, and advancing the wire electrode 1 along the machining profile of the work 8 until it reaches the front end of discharge machining. That is, the wire feeding portion must be returned to the start hole 26 every time when the wire electrode 1 breaks. The wire electrode 1 that has passed through the start hole 26 of the work 8 is then advanced along the worked profile that has been discharge machined until it reaches the point from where the machining is to be started. Therefore, the time is waisted and the discharge machining is not efficiently carried out.
A wire breakage detecting device has been proposed for use with the conventional electric discharge machines to detect the breakage of wire in case the wire electrode is broken when it is being fed during the discharge machining. For instance, a wire breakage detecting device used for an electric discharge machine has been disclosed in Japanese Utility Model Laid-Open No. 106128/1989 filed by the applicant of the present application. This wire breakage detecting device will now be briefly described with reference to FIG. 8. Breakage in a wire electrode 1 is detected by a fiber-type photoelectric sensor 85, and the machining power source is cut off in response to a breakage signal of the fiber-type photoelectric sensor 85 to stop the electric discharge machining. The sensor 85 is installed being opposed to a wire that serves as the wire electrode 1 positioned in the wire electrode feeding system, exhibits a function to detect breakage in the wire, and sends a breakage signal to a controller which cuts off the machining power source in response to the breakage signal to stop the electric discharge machining. The sensor 85 may be installed at a position between a roller 87 on the upstream side of an upper wire head 86 and a brake roller 88 arranged on the upstream side of the roller 87 and where no deflection takes place in the wire that serves as the wire electrode 1. In addition to making it possible to smoothly carry out the operation of the electric discharge machine unattended, the automatic wire feeder makes it possible to automatically connect the wire together again even when it is broken during the machining, and exhibits a try-again function which tries to connect the wire together repetitively even when the feeding of wire results in failure until the wire is connected together. The wire electrode 1 supplied from a source bobbin 89 by the automatic wire feeder passes through a variety of rollers, through a dies guide of a wire head 86, subjected to discharge machining the work, passes through a dies guide of a wire head 90 and rollers, taken up by a take-up roller 91, and is taken up by a take-up reel or is discharged onto a used wire storage unit.
Another wire breakage detecting device has been disclosed in Japanese Patent Application No. 221416/1988 that was filed by the applicant of the present application. The wire breakage detector device is arranged in the wire electrode running system that guides the wire electrode, and comprises a roller that rotates in response to the running of the wire electrode, a revolution sensor which is opposed to the roller to detect the number of revolutions of the roller, and a controller which detects whether the revolution signals of the roller as detected by the revolution sensor lie within a predetermined range or not, and which cuts off the machining power source in response to an abnormal signal in the detect signals to interrupt the machining of the work.
Japanese Patent Publication No. 16771/1987 discloses a further line breakage detecting device which comprises a first detecting means for detecting an average value of peaks in the machining voltage, a second detecting means for detecting an instantaneous value of said machining voltage, and a comparator means which compares the value detected by the first detecting means with a value detected by the second detecting means, and which produces a signal for cutting off said power source when the instantaneous value becomes greater than the average peak value by more than a predetermined amount.